The present invention relates to optical substrates, particularly to optical substrates that contain coated reflective optics, and more particularly to a pedestal type optical substrate that maintains acceptable figure before or after coating, is not sensitive to mount disturbance loads, and demonstrates high dynamic stiffness.
Optics for extreme ultraviolet (EUV) lithography imaging systems, for example, require coated reflective optics that are precisely figured at the angstrom level. In such reflective optics, the optical substrates are first figured, then coated with a film of material that is reflective in the ultraviolet wavelengths. Unfortunately, the coated film has a residual stress that causes the substrate to deform from its desired figure. It has been shown that for a disk-like optical substrate, the nonspherical component of the figure distortion decreases as the aspect ratio (width divided by height) of the substrate increases. Thus, while thinner substrates deform more due to the coating, the deformation is primarily spherical. In many cases an imaging system is relatively insensitive to spherical changes in an individual optic. Thus, a thinner substrate reduces the negative effect of coating residual stress.
The coated optics must be assembled into the imaging system using some form of mechanical attachment. The mechanical attachment, referred to as the optical mount, must physically attach to the substrate. This attachment provides a mechanism through which undesired figure distorting forces and moment can be applied to the substrate. There are many sources of these disturbance inputs, including system temperature changes, distortion of their imaging system structure, and residual stress due to the mounting process. The sensitivity of the optics surface figure is directly related to the mechanical stiffness of the substrate. An infinitely stiff substrate would not deform due to these inputs. A realistic, disk-like substrate will always demonstrate a level of sensitivity to these mount induced disturbance loads. The lower the aspect ratio of a disk-like substrate, the lower the optic's sensitivity to disturbance loads. Thus, a thicker substrate reduces the sensitivity of a substrate to disturbance loads.
Frequently, there is a requirement that an optical substrate demonstrates a high degree of dynamic stiffness. A low aspect ratio substrate generally provides for more favorable (higher) dynamic stiffness.
The problem is that while a thinner substrate reduces negative coating effects, it may not provide the necessary stiffness and insensitivity to mount disturbances. The present invention, which involves a pedestal substrate, addressees the need to have an optical substrate that maintains acceptable figure before or after coating, is not sensitive to mount disturbance loads, and demonstrates high dynamic stiffness. The pedestal substrate of the present invention comprises the basic components: 1) a disk-like optic or substrate section, the top surface of which is to be coated with reflective material, 2) a disk-like base section, and 3) a connecting section between the base and optic or substrate sections. These three sections may be formed as a monolith from a solid piece of material, the optic or substrate section and the connecting section may be formed from a solid piece, or the three sections and being formed individually and secured together. The pedestal substrate approach of the present invention permits the optic component designer to independently control the effects of residue coating film stress, mount disturbance loads, and substrate dynamic stiffness.